The present invention relates to a carbon monoxide sensing element and more particularly it relates to a carbon monoxide sensing element capable of sensing selectively lower concentrations of carbon monoxide within a quicker response time with better SN and temperature-resistance variation ratios than the prior art. The present invention also relates to a method of making such an element.
Heretofore there have been many reports on the study of CO gas sensing elements and they have also been described in various patent documents. Of such patent literature the following examples can be listed;
Japanese Patent Publication No. 45-38200 (1970) discloses a warning apparatus for reducing gases such as H.sub.2 O, C.sub.2 H.sub.2 comprising a reduction type semi-conductor such as ZnO or SnO.sub.2 as a sensing element, the sensing ability of which is connected through an amplifier or directly to a warning display and the sensing element is heated to eliminate the effects of humidity on the response of said element and to effect the rapid adsorption/desorption of the reducing gas on said element. This apparatus operates for example, in excess of 500.degree. K. for any organic gas such as C.sub.3 H.sub.6, and in excess of 420.degree. K. for H.sub.2, CO or city gas containing same.
Japanese Patent Publication No. 47-38840 (1972) discloses a gas responsive element comprising a reduction type semi-conductor metal oxide base material, a catalyst selected from at least one of a group consisting of Pa, Ag, Fe, Co, Ni, Mo and Cu with the prerequisite that said catalyst differs from said base material, and a restoring agent selected from a group consisting of MgO, PbO and CaO. As the semi-conductor material metal oxide semi-conductors such as ZnO, SnO, Fe.sub.2 O.sub.3, CdO, V.sub.2 O.sub.5, TiO.sub.2 and TaO.sub.2 are named. This publication also describes in the sole example that the heating of said gas responsive element is necessary for actual use. The example illustrates application of this element to city gas.
Japanese Patent Publication No. 53-23195 (1978) describes an apparatus for sensing the concentration of carbon monoxide comprising a sensing element composed of metal oxide semi-conductor material, said sensing element being produced by adding to stannic oxide or a salt of tin decomposing into stannic oxide upon being fired, at least one selected from salts of platinum and platinum black and firing the resultant composite, and means for changing the temperature of said element, the detection of the concentration of carbon monoxide being effected by the particular temperature at which an abrupt dramatic change of electric resistance in said element occurs. This sensing element selectively responds to carbon monoxide in a gaseous atmosphere, but it also requires heating means upon measurement.
U.S. Pat. No. 4,000,089 describes an element which can detect selectively carbon monoxide from other reducing gases contained in air. The element has a composition comprising stannic oxide as a basic material, platinum black as a catalyst and if necessary, clay, ferric oxide or glassy substances as a calcining agent. FIG. 4 shows a graphic chart in which if the temperature is fixed at 60.degree. C. the sudden change of electric resistance value occurs at a carbon monoxide content of 500 ppm in air and it is insensitive to contents below 500 ppm carbon monoxide. Further this patent shows the electric resistance at 25.degree. C. of reducing gases including CO of 1000 ppm in air.
Japanese Patent Laid Open 56-49950 (1981) discloses a sensing element comprising a semi-conductor wafer composed of a base of stannic oxide to which at least a predetermined amount of Pt is carried and having a platinum pin electrode in pin contact with the semi-conductor wafer as one electrode. This laid open publication also discloses the optional doping of a trace amount of antimony for stabilizing the temperature properties.
Further, in Japanese Patent Laid Open No. 53-143298 which partly corresponds to U.S. Pat. No. 4,251,225, a SnO.sub.2 -Sb.sub.2 O.sub.3 -Pt type sensing element for carbon monoxide composed of stannic oxide (SnO.sub.2), antimony trioxide (Sb.sub.2 O.sub.3) and platinum (Pt) which has a suppressed temperature dependence without impairing the selectivity for carbon monoxide (CO).
This SnO.sub.2 - Sb.sub.2 O.sub.3 -Pt type sensing element for carbon monoxide requires no heater for heating the element, but has a disadvantage in that low concentrations of CO gas cannot be detected.
However, such gas detectors have many disadvantages such as low selectivity for CO detection, the need for heating the gas sensing element, the high temperature dependence and the like.
The present inventor carried out various tests in view of the above point and succeeded in achieving a CO gas sensing element which by using antimony oxychloride (SbOCl) in place of the compositional constituent Sb.sub.2 O.sub.3 can detect lower concentrations of CO gas, has superior response to gas and also excellent thermal characteristics in comparison to the prior SnO.sub.2 - Sb.sub.2 O.sub.3 - Pt type gas sensing elements and further it does not require a heater.